In the description of the background of the present invention that follows reference is made to certain structures and methods. Such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
The term “nanostructure” material is used by those familiar with the art to designate materials including nanoparticles such as C60 fullerenes, fullerene-type concentric graphitic particles; inorganic and organic nanowires/nanorods composed of either single or multiple elements such as Si, Ge, metals, oxides such as SiOx, GeOx; carbides such as silicon carbides; nitrides, borides, or hollow nanotubes composed of either single or multiple elements such as carbon, BxNy, CxByNz MoS2, and WS2. One of the common features of nanostructure materials is the dimension of their basic building blocks. A single nanoparticle or a nanotube or a nanowire has a dimension that is less than 1 micron in at least one direction. These types of materials have been shown to exhibit certain properties that have raised interest in a variety of applications and processes.
U.S. Pat. No. 6,280,697 to Zhou et al. entitled “Nanotube-Based High Energy Material and Method,” the disclosure of which is incorporated herein by reference, in its entirety, discloses the fabrication of carbon-based nanotube materials and their use as a battery electrode material.
Application Ser. No. 09/296,572 entitled “Device Comprising Carbon Nanotube Field Emitter Structure and Process for Forming Device” the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon nanotube-based electron emitter structure.
Application Ser. No. 09/351,537 entitled “Device Comprising Thin Film Carbon Nanotube Electron Field Emitter Structure,” the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon-nanotube field emitter structure having a high emitted current density.
U.S. Pat. No. 6,277,318 to Bower et al. entitled “Method for Fabrication of Patterned Carbon Nanotube Films,” the disclosure of which is incorporated herein by reference, in its entirety, discloses a method of fabricating adherent, patterned carbon nanotube films onto a substrate.
U.S. Pat. No. 6,334,939 to Zhou et al. (application Ser. No. 09/594,844) entitled “Nanostructure-Based High Energy Material and Method,” the disclosure of which is incorporated herein by reference, in its entirety, discloses a nanostructure alloy with alkali metal as one of the components. Such materials are described as being useful in certain battery applications.
Application Ser. No. 09/679,303 entitled “X-Ray Generating Mechanism Using Electron Field Emission Cathode,” the disclosure of which is incorporated herein by reference, in its entirety, discloses an X-ray generating device incorporating a nanostructure-containing material.
Application Ser. No. 09/817,164 entitled “Coated Electrode With Enhanced Electron Emission And Ignition Characteristics” the disclosure of which is incorporated herein by reference, in its entirety, discloses an electrode including a first electrode material, an adhesion-promoting layer and a carbon nanotube-containing material disposed on at least a portion of the adhesion promoting layer, as well as associated devices incorporating such an electrode.
Application Ser. No. 09/881,684 entitled “Method of Making Nanotube-Based Material With Enhanced Field Emission” the disclosure of which is incorporated herein by reference, in its entirety, discloses a technique for introducing a foreign species into the nanotube-based material in order to improve the emission properties thereof.
As evidenced by the above, nanostructure materials, such as carbon nanotubes, possess promising properties. Carbon nanotubes (CNTs) are one type of nano objects. CNTs are cylindrical carbon structures with a length between 0.1 μm and 100 μm and a diameter between 0.4 nm and 50 nm (see, e.g. M. S. Dresselhaus, G. Dresselhaus, and P. Avouris, eds. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications. Topics in Applied Physics. Vol. 80. 2000, Springer-Verlag). CNTs can have either a single graphite shell per nanotube in which case CNTs are called single-wall carbon nanotubes (SWNTs). CNTs may also have concentric multi-shell graphite structures in which case CNTs are called multi-wall carbon nanotubes (MWNTs). Carbon nanotubes have exceptional mechanical properties with high elastic modulus, high ductility, high electrical and high thermal conductivity, thermal stability and chemical stability. CNTs are excellent electron field emitters since CNTs have a large aspect ratio and a sharp tip. (See, e.g. P. M. Ajayan and O. Zhou, in “Topics in Applied Physics, 80,” M. S. Dresselhaus, G. Dresselhaus, and P. Avouris, Editors. 2000, Springer-Verlag). In particular, carbon-nanotube materials exhibit low emission threshold fields as well as large emission current densities. Such properties make them attractive electron field emitters for microelectronic applications, such as lighting elements, field emission flat panel displays, gas discharge tubes for over voltage protection and x-ray generating devices. Other applications of carbon nanotubes include but limited to: sensors, composites, shielding materials, detectors, electrodes for batteries, fuel cells, small conduction wires, small cylinders for storage, etc.
Carbon nanotubes, nanowires and nanorods, nanoparticles are typically fabricated by techniques such as laser ablation, arc discharge, and chemical vapor deposition methods. In some cases they can also be made via solution or electrochemical synthesis. However, in most cases, the as-synthesized materials cannot be utilized without further processing. For example, carbon nanotubes produced by the laser ablation and arc discharge techniques are in the form of porous mats and powders. Device applications require assembling these nano objects into ordered, patterned films, membranes, crystals on desired supporting surfaces and the pre-determined locations. In addition, it is often advantageous to assemble elongated nano objects such as the carbon nanotubes into orientationally ordered macroscopic structures which provide properties such as anisotropic electrical, mechanical, thermal, magnetic and optical properties.
The conditions used to assemble the nano objects need to be compatible with the conditions used for device fabrications. For example, nano objects which are to be used as field emission cathodes in field emission display should not have a fabrication temperature which exceeds the melting point of glass substrates (about 650° C.). Also, the temperature should be substantially lower when supporting surfaces of the nano objects are polymer. For such applications, direct growth of nano objects using chemical vapor deposition (CVD) techniques are not feasible since CVD techniques generally require relatively high temperatures (800° C.–1200° C.) as well as reactive environments. In addition, CVD techniques often result in defective multi-wall carbon nanotubes.
As such, a more desirable approach for fabrication of macroscopic structures of nano objects for applications is post-processing which includes synthesis of nano objects by arc discharge, laser ablation, or chemical vapor deposition techniques and assembly of these “pre-formed” nano objects into a macroscopic structure. Post-deposition processes that have been employed in the past include screen printing. (see, e.g. W. B. Choi, et al., Appl. Phys. Lett., 75, 3129 (1999)), spraying, and electrophoretic deposition (see, e.g. B. Gao et al. Adv. Mater., 13 (23), 1770, 2001) However, such techniques pose certain drawbacks. For instance, screen printing includes admixture of pre-formed nano objects with an organic or inorganic paste in order to form a thick paste. The thick paste is then placed on a substrate. After placement of the thick paste onto the substrate, the organic binder resides at an exposed surface of the paste. Therefore, an additional step is required to expose the nano objects within the thick paste. Typically, a plasma etching process or similar chemical process is used to expose the nano objects. In addition, the use of thick paste limits the size of a structure can be formed. In general it is difficult to form structures less than 20 microns–50 microns using the screen printing methods. Furthermore, screen printing methods requires considerable amount of materials. Spraying can be inefficient and is not practical for large-scale fabrication. Neither of these processes can control the orientation of the nano objects.
Therefore, a need exists for a process/method to assemble nano objects with a controlled structure, morphology, thickness, orientation, and ordering. In addition, a need exists for a method that operates at mild conditions acceptable for device fabrications. In addition, a need exists for an efficient and scalable assembly process.